1. Field of the Invention
The present invention relates to a flat cathode ray tube, and more particularly, to a frame assembly in a flat cathode ray tube, which can prevent howling, a mask vibration, occurred when a frame assembly and a shadow mask are resonant
2. Background of the Related Art
Referring to FIGS. 1 and 2, the flat cathode ray tube is provided with a panel glass 1, a shadow mask 3 fixed to a rear face of the panel glass 1 with a pre-tension having numerous apertures of circular or slot forms for selection of colors, a magnetic shield 9 fixed to an inside surface of the panel glass 1 for shielding electron beams 6 so that a path of the electron beams 6 are not deviated by an external geomagnetic field or a leakage magnetic field, a funnel glass 2 fixed to the panel glass I with frit glass having a neck portion formed as one unit in a rear portion thereof, an electron gun sealed in the neck portion of the funnel glass 2 for emitting R, G, B three color electron beams 6, and a deflection yoke 5 DY mounted to surround an outer surface of the neck portion for deflecting the electron beams 6.
In the meantime, since the flat cathode ray tube is susceptible to an external impact owing to an internal high vacuum, the panel glass 1 is designed to withstand the atmospheric pressure. And, there is a reinforcing band 11 strapped around a skirt portion of the panel glass I for spreading a stress in the cathode ray tube under a high vacuum to secure an impact resistance.
In the operation of the flat cathode ray tube, the electron beams 6 from the electron gun in the neck portion of the funnel glass 2 are made to hit onto fluorescent material surface 4 on an inside surface of the panel by an anodic voltage provided to the cathode ray tube, when the electron beams 6 are deflected in an up, down, right, or left direction by the deflection yoke 5 before the electron beams 6 reach to the fluorescent material surface 4. There are 2xc2x74xc2x76 polar magnets 10 in rear of the neck portion for correcting a path of travel of the electron beams 6 so that the electron beams 6 can hit onto an intended fluorescent material and prevent occurrence of defective color purity.
In the meantime, referring to FIGS. 2 and 3, the shadow mask 3 in the flat cathode ray tube is provided with an effective area 3 a having the numerous apertures, and an edge portion 3b without the apertures for reinforcing a strength of the effective area 3a. The shadow mask 3 is prestressed in up and down directions with a tension P1. That is, in the related art, the application of the tension P1 to the shadow mask 3 by the frame assembly designed to have a high rigidity makes the shadow mask 3 resonant at a high frequency, thereby preventing howling in which the shadow mask 3 vibrates. The frame assembly 7 has a main frame 7a fitted to both ends of the shadow mask 3 directly, and a subframe 7b fitted across the main frame 7a. There is a spring 8 fitted to the main frame 7a for fixing the frame assembly on an inside surface of the panel glass 1.
However, the welding of top and bottom edges 3b of the shadow mask 3 to the main frame 7a to exert the tension to the shadow mask 3 causes an intense vibration of the shadow mask 3 owing to resonance with the frame assembly 7 when an external vibration is transmitted to the shadow mask 3, resulting in a beam landing error, that causes howling, in which the electron beams from the electron gun can not hit onto the fluorescent material film exactly. That is, as shown in FIGS. 4xcx9c5, while natural frequencies of the shadow mask 3 are distributed closely at a frequency range over 150 Hz continuously, as shown in FIGS. 6 and 7, a first natural frequency of the frame assembly occurs at 120 Hz in a twisting mode, and a second natural frequency of the frame assembly occurs at 170 Hz in a shearing mode. Consequently, though the shadow mask 3 is not resonant with the frame assembly 7 at the first vibration mode of twisting mode, as shown in FIGS. 4 and 7, the shadow mask 3 is resonant with the frame assembly 7 at the second vibration mode of shearing mode, because natural frequencies of the shadow mask 3 and the frame assembly 7 are the same. In summary, in the related art, the intense vibration of the shadow mask 3, a color selection electrode, owing to resonance between the frame assembly 7 and the shadow mask 3 caused by an external vibration results in a beam landing error in which the electron beams from the electron gun can not hit the fluorescent material exactly, that in turn causes howling, a vibration of the picture.
Accordingly, the present invention is directed to a frame assembly in a flat cathode ray tube that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a frame assembly in a flat cathode ray tube, in which a second moment of inertia of a subframe forming a frame assembly with a main frame is optimized for preventing occurrence of howling of the shadow mask, a vibration of the shadow mask, caused by resonance between the frame assembly and the shadow mask, and minimizing weight of the frame assembly.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the frame assembly in a flat cathode ray tube, the flat cathode ray tube includes a panel glass having a fluorescent material coated on an inside surface, a funnel glass fixed to rear of the panel glass having a neck portion formed as one unit with an electron gun sealed therein for emission of electron beams toward the fluorescent material, a deflection yoke fitted on an outer circumference of the neck portion for deflection of the electron beams emitted from the electron gun, a shadow mask fitted to an inside surface of the panel glass having a plurality of apertures for selecting colors, and a frame assembly having a main frame fitted to the shadow mask and a subframe connecting both ends of the main frame, wherein a ratio of second moment of inertia(Ixx/Izz) of the subframe is designed to be within 0.5xcx9c2.7 for avoiding a resonance between the frame assembly and the shadow mask, thereby preventing occurrence of howling, effectively.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.